In the past few years, parabolic reflectors of nine, twelve feet, etc., in diameter, have been employed at sites within the North American hemisphere for receiving television signals by aiming the antenna at a chosen geosynchronous satellite within an equatorial satellite track and in particular with respect to a limited sector of that track, bearing a series of closely spaced satellites.
The importance and necessity of accurately aligning the focal axis or boresight of the antenna with a chosen satellite can be appreciated when it is understood that the satellites are beaming television signals to the continental United States, a broadcasting distance of 25,000 miles or so, with a power of only five watts. Further, along their orbital track in the plane of the equator, they are separated by as little as four degrees and are likely to be more closely spaced as future satellites are launched. It is only in the equatorial plane that synchronous orbit can be achieved, whereby even at a distance of 25,000 miles or so, the satellites vary their positions by no more than one-half mile. Specifically, the parabolic receiving antenna is so highly directional that as little as a one-fourth degree misdirection may result in significant loss of signal quality.
There are two basic types of mount: one is the two axis type where, by azimuth and elevation adjustment, the parabolic receiving antenna is provided with some degree of alignment between the focal axis of the antenna and the satellite selected from the group of satellites within a given sector with the aximuth and elevation adjustments being undertaken with consideration to latitude and longitude position of the antenna on the earth's surface.
While this type works satisfactorily when there is a single satellite to which the antenna is to be focussed, the industry utilizes a so-called "polar mount", which is highly favored, when any one of a number of satellites must be conveniently chosen and where the orientation of the antenna may be changed at random, depending upon the television programs broadcast by the various satellites. The function of the polar mount is to direct the antenna towards a chosen satellite by pivoting the antenna on a single axis with no further adjustment required after desired latitude and longitude presetting is effected. The polar mounts presently supplied for use with TVRO (television receive only) installations fail to do this with sufficient accuracy except at limited geographical areas where the signals are strongest and where certain errors inherent in the alignment process are the least. In locations removed from the most favorable positions, these deficiencies are overcome by the use of larger antennas, more sophisticated radio frequency amplifiers, or both. These are relatively costly solutions compared with improving the performance of the mount itself.
It is therefore a primary object of the present invention to provide a polar mount incorporating a fixture, as an attachment to or integrated with a polar mount, which fixture can be set in accordance with information related to the latitude of the antenna site which insures alignment of the mount for the antenna relative to the satellite to a known degree of accuracy and one which is much higher than that achievable in the past.
It is a further object of the present invention to provide such a polar mount, or a fixture as an attachment thereto, which makes use of information developed mathematically to provide a more sophisticated alignment procedure and a structural combination facilitated by the fixture itself which permits rapid, accurate alignment of the antenna to the satellite track and in which the fixture may be readily adjusted from data derived by way of a computer program using the mathematical formulae.
It is a further object of the present invention to provide an improved polar mount adapted physically to use the aligning fixture and which will readily maintain the resulting accuracy in service over an appreciable time period irrespective of weather conditions experienced by the antenna.